DYNAMIC UPLIFT DURING SLAB FLATTENING 1,2Federico M. Dávila and 1Carolina Lithgow-Bertelloni 1Dept. Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK 2CICTERRA-CONICET-Universidad Nacional de Córdoba, Córdoba 5016, Argentina. [email protected] Abstract Subduction exerts a strong control on surface topography and is the main cause of large vertical motions in continents, including past events of large-scale marine flooding and tilting. The mechanism is dynamic deflection: the sinking of dense subducted lithosphere gives rise to stresses that directly pull down the surface. Here we show that subduction does not always lead to downward deflections of the Earth's surface. Subduction of young lithosphere at shallow angles (flat subduction) leaves it neutrally or even positively buoyant with respect to underlying mantle because the lithosphere is relatively warm compared with older lithosphere, and because the thickened and hence drier oceanic crust does not undergo the transformation of basalt to denser eclogite. Accounting for neutrally buoyant flat segments along with large variations in slab morphology in the South American subduction zone explains along- strike and temporal changes in dynamic topography observed in the geologic record since the beginning of the Cenozoic. Our results show that the transition from normal subduction to slab flattening generates dynamic uplift, preventing back-arc marine flooding. Introduction South America is an ideal natural laboratory to study the influence of subduction morphology on dynamic topography. The present-day geometry of subduction (>3000 km length) changes radically 1 along strike (Booker et al., 2004; Gans et al., 2011) and includes two regions with flat subduction, each associated with the subduction of aseismic ridges with over thickened crust (Nazca and Juan Fernandez ridges, Fig. 1). The location and extent of flat subduction segments has varied with time (Kay and Mpodozis, 2002), allowing us to relate the time history of dynamic topography as revealed in foreland basins to secular variation in subduction zone morphology. Global and regional models of present-day South American dynamic topography and its temporal evolution (Lithgow-Bertelloni and Gurnis, 1997; Lithgow-Bertelloni and Richards, 1998; Shephard et al., 2010, 2012) disagree with the geologic record (Jordan and Allmendinger, 1986; Espurt et al., 2007; Dávila et al., 2010, 2012; Devlin et al., 2012). Though the temporal evolution in both Lithgow-Bertelloni and Gurnis (1997) and Shephard et al. (2010) imply differential uplift and subsidence from the early Cenozoic to the present, the amplitudes and locations differ substantially with respect to the geologic record and imply different dynamical mechanisms. Shephard et al. (2010) predict a large subsiding area 30-40 My that occupies almost all of northern South America including the Andes and Andean foreland that gradually uplifts dynamically as the slab migrates and steepens. The models predict subsidence in geographic disagreement with the location of the northern and southern Amazonian basins and in detail with topographic observations of uplift (e.g., Espurt et al., 2007 in Peru or Dávila et al., 2012 in Argentina). The dynamical evolution of the slab is also in stark contrast with the present-day Nazca slab morphology (Fig. 1b). At 60 My an enormous mass of flat cold material (‘slab’) occupies the entire upper mantle evolving to a more steeply dipping slab for the present-day. The slab evolution is not a short-coming of these sophisticated state-of-the-art adjoint models, but rather stems from known limitations for the backwards advection of initial conditions (Bello et al., 2014) choices regarding slab and plate boundary rheology and an initial buoyancy source derived from global tomographic models whose wavelengths, inherent damping and resolution do not capture crucial aspects of slab morphology and density structure. The latter include along- 2 strike variations in subduction angle and density structure due to age variations and thicker than normal crust. These large age variations are important for elevations within and away from the mountain belt as highlighted by Capitanio et al. (2011). Older (cooler, denser) slabs lead to faster subduction and more vigorous flow in the mantle wedge. In turn the flow exerts stronger shear tractions at the base of the overriding plate leading to crustal thickening and higher elevations. Along-strike variations in subduction morphology (e.g., Fig. 1) are apparent in South America’s foreland surface topography. In the southern part of the Peruvian foreland (10˚ S) up to >700 km from the trench we find the Fitzcarrald arch, an elevated long-wavelength bulge (Espurt et al., 2007; Fig. 1b). This Arch overlies the Peru flat slab and lies eastward from the easternmost Andean thrusts, where no thrusting or folding has been documented (Espurt et al., 2007). This segment also exposes a high- elevation intermontane broken foreland system (Devlin et al., 2012) (PBF in Fig. 1b). Further south, above the Argentine-Chilean flat-slab segment at 30-32˚ S, the distal foreland (>600 km eastward) also shows a high-elevation foreland system (Fig. 1b): the Sierras Pampeanas broken foreland (often used as a modern analogue for the US Laramide, Jordan and Allmendinger, 1986). The Sierras Pampeanas are isostatically uncompensated and have associated high-elevation depocenters >1 km above sea level (Dávila et al., 2012). These features date to the latest Miocene-Pliocene, coeval with the advent of flat subduction (Ramos, 2009). Flat subduction is also often associated with tectonic deformation like lithospheric thickening and basement thrusting, present in the Sierras Pampeanas but not on the Fitzcarrald Arch. Here we first produce high-resolution models that capture the morphology and density structure of the Nazca slab (Fig 1b) and then use them to predict the surface dynamic topography produced by the induced viscous flow (Fig. 2). We show that flat subduction has a decisive influence on the sign, amplitude, and pattern of dynamic topography. We suggest that the change from ‘typical’ subduction to slab flattening leads to dynamic uplift rather than subsidence (Fig. 3). Together with crustal deformation (Dávila et al., 2010; Dávila and Lithgow-Bertelloni, 2013) and lithospheric thickness 3 changes due to, for example delamination (Garzione et al., 2008), the dynamic uplift determines today’s total topography (Fig. 3) as suggested by observed deficits and excesses in residual topography (Steinberger, 2007). We compare our results with two independent sources, a regional residual topography model and structural reconstruction of the regional paleosol marker of the Los Llanos Formation, which developed on a flattened surface across the Miocene foreland (Fig 2 bottom right panel). Methodology We assume that the dominant driving force for mantle flow and the resulting surface dynamic topography is subducted density heterogeneity. Subducted heterogeneity, in an otherwise neutrally buoyant mantle, excites instantaneous flow that produces surface deformation, plate motions, and geoid in good agreement with a wealth of observations as shown in a number of previous studies (e.g. Lithgow-Bertelloni and Richards, 1998). An important advantage of focusing on subducted heterogeneity is that slab morphology for South America is known in detail. Our present-day Nazca slab model (Fig 1 bottom panel) is considered as representative of the slab geometry over the Neogene (~20 My to present), and derived from constraints on slab morphology from regional seismic and magneto-telluric surveys (Gutscher, 2002; Booker et al., 2004; Gans et al., 2011). This episode temporally matches the formation of the foreland landscapes in Peru and Argentina (Espurt et al., 2007; Dávila et al., 2012) described above. Some uncertainty exists over the exact timing of the arrival of the flat slab, which could be as late as 12-11 Ma (Ramos and Folguera, 2009) though the vas majority of the literature supports a date closer to 20 Ma (Dávila et al., 2004). The subduction angles vary along strike from flat (0˚) to a typical (30˚) for South America and the density contrasts (∆) of the slab (with respect to the mantle) are controlled by the slab age, the inferred crustal thickness and the Benioff zone geometry (Fig. 1 bottom panel). The net buoyancy 4 difference between normal and oceanic lithosphere with overthickened crust is largely determined by the thickness of the basaltic layer and that of the depleted harzburgitic mantle (c.f. Cembrano et al. 2007, Arrial and Billen, 2013). From the crustal thicknesses and compositions across the Argentine and Peruvian flat-slab segments (~20 km, Gans et al., 2011; Bishop et al., 2013) and the Nazca ridge offshore (~17-18 km, Woods and Okal, 1994) and the self-consistent thermodynamics of Stixrude and Lithgow-Bertelloni (2011), we estimated an integrated density contrast across the modern flat slab of ~10 kg/m3 less than ambient mantle (c.f. Appendix). To be conservative we modeled flat-slab segments with neutral buoyancy ∆ = 0 (Fig. 1, see Appendix for further detail), though we also show results for the former. Typical density contrasts for subducting slabs are as high as 80 kg/m3 greater than ambient mantle. Our highest density contrast is 50 kg/m3 for the oldest subducting lithosphere. The resulting Nazca slabs have a resolution of 1°x1°, considerably higher than previous models, though density and geometric variations occur on average every 400-500 km (Fig. 1 bottom panel). The new high-resolution Nazca slabs are embedded in an existing global slab model (Lithgow-Bertelloni and Richards, 1998) and expanded to spherical harmonic degree and order 80. We compute the instantaneous flow and resulting dynamic topography (c.f Appendix for technical details) induced by these density fields in a spherical shell to degree and order 50 for a mantle with a radially layered viscosity that best matches the observed geoid (Lithgow-Bertelloni and Richards, 1998) (Appendix, Fig. Ap2). The density and flow field expansions correspond to wavelengths between 500-800 km, comparable to the variations in slab morphology and density.